I. FIELD OF THE INVENTION
This invention relates to the recovery of non-ferrous metals, particularly aluminum, from dross.
II. DESCRIPTION OF THE PRIOR ART
Dross is a material which forms on the surface of molten non-ferrous metal during remelting and metal holding and handling operations when the molten metal is in contact with a reactive atmosphere. Dross normally consists of metal oxides entraining a considerable quantity of molten free (unreacted) metal, and for economic reasons it is desirable to extract the free metal before discarding the residue.
The traditional processes for recovering the free metal generally involve one or several of the following steps:
1. Cooling of the dross using either a mechanical cooler or a dross room (where the dross is spread out over a floor). Some metal recovery may take place at this stage by drainage of the metal from the dross, possibly assisted by mechanical agitation.
2. Transportation of the cooled dross to a dross treatment plant.
3. Crushing and screening of the cooled dross, followed by elimination (usually by dumping) of the fine fraction which consists mainly of oxides and which may be dangerous and disadvantageous to handle in subsequent steps.
4. Heating of the larger dross fractions in the presence of a molten salt bath in order to remelt the metallic fraction and cause the molten droplets to coalesce.
Step 4 is normally carried out in a rotary furnace. The dross fractions and salt mixture (normally a 50:50 mixture of NaCl and KCl to which a small quantity of fluoride salt is optionally added in order to lower the melting point of the salts and to increase the wettability of the solid oxides) are charged and heated to a temperature above the melting point of the metal using direct flame burners. The furnace is then rotated at a suitable rate of speed to obtain a tumbling or cascading action of the mixture.
While salt bath furnaces are acceptably efficient and high metal recoveries can be obtained, they have serious disadvantages, including those listed below:
(i) Salt fumes generated by the molten salt (which can be copious if a burner is directed onto the salt bath) are very corrosive both within the plant and to the external environment. Elaborate and expensive exhaust gas collection and cooling equipment and dust collection systems are required.
(ii) The residual salt cake with dross impurities is very polluting because the salts are water soluble and are readily leached out of dump sites. They must therefore be dumped only in specially prepared land-fills, which is a severe problem because of the large quantities produced (approximately two parts by weight of residue are generated for each part by weight of recovered metal).
(iii) Salt bath furnaces are thermally inefficient because
(a) a fossil fuel heating system is employed for which heat recuperation is impossible due to the corrosive nature of the exhaust gases; PA1 (b) approximately one part by weight of salt must be melted for each two parts by weight of dross.
Step 3 also gives rise to disposal problems because the dross fines are also rather toxic and difficult to handle.
Processes have recently been proposed to avoid these problems. For example, it has been proposed to extract the liquid metal from dross by mechanical compression of the hot dross removed directly from a furnace. This process requires expensive equipment and high dross temperatures and is limited by these and other factors to relatively large scale operations. Moreover, this approach does not directly address the disposal problems because the residues still contain a large quantity of free metal.
It has also been proposed (in the case of aluminum dross) to induce and maintain burning or thermitting of the dross under controlled conditions by working the dross in an inclined rotary barrel open to the atmosphere, thus permitting a certain portion of the metal content to be consumed in order to recover the remainder. This method has the drawback of being technically complicated to operate and of producing voluminous polluting fumes. In a modified version of this process, the barrel is closed and aluminum chloride vapour is introduced in order to control oxidation. However, the problems of safety and fume control that arise when the barrel is opened are even more acute than those arising from the original process.
There is consequently a need for a more acceptable way of recovering non-ferrous metals from dross that avoids the safety and pollution concerns and can be operated relatively economically.